Our German license plates are usually a combination of one up to three letters of the city/county where the vehicle is registered (Like M for Munich or BIT for Bitburg), followed by one or two letters you can choose (people often like their initials here) and then it is a one to four digit number you can also choose, unless already occupied, often the birthdate of the owner or maybe 911 when it is about a Porsche. Or if my car was registered in Recklinghausen, I might choose RE - NE xxx.

From Wiki:

"Prohibited combinations:

Various combinations that could be considered politically unacceptable — mainly due to implications relating to Nazi Germany — are disallowed or otherwise avoided. The district Sächsische Schweiz uses the name of its main town, Pirna, in its code PIR, to avoid the use of SS, the name of the paramilitary organization; similarly SA is also unused. In 2004 in Nuremberg, a car owner was refused a number plate beginning N-PD because of the connection to the political party the NPD.[citation needed] The combinations STA-SI, HEI-L and IZ-AN are also avoided, to avoid association with Stasi, with the Nazi salute and with NAZI backwards.

Banned combinations also include the Nazi abbreviations HJ (Hitlerjugend, Hitler Youth), NS (Nationalsozialismus, National Socialism), SA (Sturmabteilung), SS (Schutzstaffel) and KZ (Konzentrationslager, concentration camp). Some registration offices have overlooked this rule by mistake, however, and there are a few cars registered carrying prohibited codes, such as B-SS 12. Some counties also allow these combinations if they are the initials of the owner (e.g., Norbert Schmidt might be able to get XX-NS 1234), but in this case, if the car is sold and re-registered in the same county by the new owner, the number can be changed (otherwise the number stays with the car until it registered in a different area). However, the combination HH (used for Heil Hitler) is used for the city of Hamburg."